Two types of rotary encoders are known. The first type concerns end-of-shaft encoders which are placed at the end of an axis and the second type concerns hollow-shaft encoders which are placed anywhere along the axis of the shaft.
These encoders suffer all of a main drawback which lies in the fact that the radial position of the said axis cannot be determined with high accuracy. The eccentricity of an axis can so not be determined with accuracy. This results also in a limitation of the precision of the determined angular position.
It is known to use a technique for decreasing the eccentricity, which is based on a visual inspection by humans using a microscope and moving a reference code-disc manually. In the final position, the code-disc is fixed by glue. This is a very expensive procedure which increases the costs of accurate encoders. Furthermore, the theoretical resolution limit of an optical microscope is not high enough to reduce the eccentricity sufficiently.
Other expensive techniques, using multiple sensor arrangements are also known. In such techniques, three tangential measurements are necessary for obtaining eccentricity compensation. The prices and dimensions of such encoders are very high because they need three sensors and to combine their output. In addition, multiple sensors lead to higher power consumption, and higher heat dissipation.